Nonaqueous electrolyte secondary batteries (typically lithium ion batteries) have recently gained importance as high-output power sources for installation on vehicles and power supplies for power storage systems since these batteries are light-weighted and can provide high energy density. Various improvements have also been made to increase further the capacity and energy density of the batteries.
In the nonaqueous electrolyte secondary battery, a separator interposed between a positive electrode and a negative electrode is provided with a function of preventing short circuit caused by contact between the positive electrode and the negative electrode (short circuit preventing function) with the object of ensuring safety of the battery and the device where the battery is installed. In addition to the short circuit preventing function, the separator also increases the resistance by shutting down the ion conduction path when the temperature inside the battery reaches a predetermined temperature region (typically, the softening point or melting point of the separator). The separator is also provided with a function of stopping charge and discharge and preventing thermal ranway of the battery by such an increase in the resistance (shutdown function). In a typical separator, the melting point of the resin such as a polyolefin, which is a constituent material, is a shutdown temperature, and where the separator reaches this temperature, fine pores of the separator are closed by melting or softening, and the resistance is increased.
Various modes of the shutdown function of such nonaqueous electrolyte secondary batteries have been suggested. For example, Patent Literature 1 discloses a separator constituted by a porous film including a resin with a melting point within a range from 80° C. to 130° C., filler particles, and a porous substrate. It is indicated that with such a configuration, the separator can maintain stably the shape even in a high-temperature state above the melting point (shutdown temperature).
Patent Literature 2 discloses the configuration in which the positive electrode of a nonaqueous electrolyte secondary battery includes a binder together with a polymer compound with a melting point of 90° C. to 130° C. and a heat of melting equal to or greater than 30 μg as a heat absorber. It is indicated that with such a configuration, even when the Joule heat is generated by a short circuit, this heat is substantially absorbed by the heat absorber contained in the positive electrode active material layer as the heat of melting and, therefore, the increase in battery temperature can be inhibited.